According to a known and widespread grafting method, radical groups are introduced into the chain of a base polymer, e.g., with the aid of high energy radiation, which causes grafting by free radical chain polymerization.
However, the use of radiation-induced grafting for the surface modification of formed bodies according to known methods not only requires the use of expensive apparatus but also has disadvantages in the quality of the products obtained. The effects of both the radiation and the grafting cannot be limited to the surface of the formed body. When grafting also occurs in the interior of the formed body, both the polymer's dimensions and its bulk properties change.
Other methods of graft copolymerization are based on activation under radical formation by means of strong oxidizing agents such as the grafting onto cellulose or polyamides in the presence of Ce.sup.IV salts. This method can only be used at very low pH, namely, at pH&lt;2, because a hydrolytic precipitation of the Ce.sup.IV salt otherwise occurs; at the same time there is the danger of a hydrolytic degradation of the polymer at such low pH. Moreover, even in this method the grafting cannot be limited to the surface of the polymer.
Still other methods of graft polymerization are based on chain transfer with homopolymerization of the monomer being induced by a radical initiator in the presence of the base polymer to be grafted, in which instances high temperatures must be used as a rule in order to effect the decomposition of the initiator. In addition to the requirement of high temperatures, a further disadvantage of this method is the fact that basically only a part of the monomer used becomes active for the grafting, leaving the remainder to accumulate as homopolymer. As a result, not only is the yield reduced, but it is also necessary to separate homopolymer from the grafted body.
It is also known that polyamides can be grafted with acrylamide or acrylonitrile as monomers in such a manner that in a first step the hydrogen atoms on the nitrogen atoms are replaced by chlorine atoms. The halogen-substituted polyamides are then converted by hydrazine or Fe.sup.II salts back into the initial polymer, during which a radical transitional state appears on the nitrogen. In the presence of monomeric acrylamide or acrylonitrile a radical chain polymerization takes place on the nitrogen atom in the same manner as in other redox-initiated polymerization methods. This reaction is described, e.g., by K. V. Phung and R. C. Schulz in Makromolekulare Chemie, pages 1800-1825 (1979), and was used to demonstrate the above-mentioned radical transitional state during the reduction.
Another work which deals with the grafting on N-halogenated polyamides described the initiation by metalcarbonyls. See C. H. Bamford, et al. in J. Polym. Sci., part C, pages 419-432 (1968). The last-mentioned process for grafting following chlorination of the base polymer is also used for surface grafting of nitrogen-containing membranes and in particular microporous membranes with the nitrogen on the polymers containing a hydrogen atom substitutable by halogens (cf. WO 91/03310 and EP-A-90 913 482.7, as well as DE-A1-39 29 648.2). A similar method is also used for the surface grafting of formed bodies from such polymers, wherein the chlorination of the polymer does not occur but the grafting is carried out in the presence of carbon tetrachloride (cf. WO 91/03506 and EP-A-90 913 193.0, as well as DE-A1-39 29 647.4). The disadvantages of these two grafting methods are that they can only be used with a certain class of base polymers and they require careful monitoring of the grafting process in order to limit the grafting to the surface of the base polymer.
The object of the present invention is to provide novel grafted formed bodies and a method of grafting on formed bodies of any desired polymer, which method is simple to carry out while avoiding grafting in the interior of the formed-body polymer, even when grafting on large amounts of a graft polymer. It is also an object of the invention to avoid disadvantageous changes in the dimensions and bulk properties of the formed-body polymer.